Operational characteristics of injection lasers under high speed modulation are of interest in optical fiber communication systems. Characteristics which are of particular interest for wideband single longitudinal mode fiber transmission are dynamic spectral behaviors such as frequency chirping and transient gain peak shifting in the transient regime and spectral envelope broadening at the occurrence of multiple longitudinal modes. Control of these dynamic spectral characteristics and others are important to achieving sufficient mode selection for single longitudinal mode operation under high speed operation.
Several approaches are known for achieving longitudinal mode selection in lasers. The approaches, excluding the use of a built-in grating for feedback, are as follows: short cavity laser, external cavity laser, and two-section (coupled cavity) laser. Each approach is described in more detail below.
Short cavity lasers employ an optical cavity which has a cavity length of approximately 30 to 80 microns. This cavity length is at least five or six times shorter than conventional optical cavity lengths. Mode selectivity of the short cavity laser arises from a much larger longitudinal mode separation and a larger gain difference between adjacent modes than in conventional lasers. Short cavity lasers are described in articles by T. P. Lee et al., IEEE J. Quantum Electron., QE-18, p. 1101 (1982), and C. A. Burrus et al., Electron. Lett., Vol. 17, p. 954 (1981).
External cavity lasers are comprised of a combination of a long optical cavity, cleaved laser and an external reflector. The reflector and a cleaved facet of the laser form an external cavity resonator which is, in general, approximately as long as the optical cavity of the laser. Diffraction losses occur in the external cavity resonator because the propagation medium is air. Mode selectivity of this combination arises from modulation of the loss in the coupled resonator including the laser and the external cavity resonator as a function of frequency. External cavity lasers have been described in articles by K. R. Preston et al., Electron. Lett., Vol. 17, p. 931 (1981); D. Renner et al., Electron. Lett., Vol. 15, p. 73 (1979); C. Voumard et al., Opt. Commun., Vol. 13, p. 130 (1975); and D. A. Kleinman et al., BSTJ, Vol. 41, p. 453 (1962).
Two section and other multiple section lasers employ a corresponding number of monolithic laser cavities abutting each other. In this type of laser, the cavities are waveguiding regions which are controllable via current biasing. In general for two-section lasers, the sections are comprised of a long section and a short section. Mode selectivity results from modulation of the loss of the laser cavities as a function of frequency. Multiple section lasers have been described in U.S Pat. No. 3,303,431 issured to A. B. Fowler on Feb. 7, 1967 and in articles by L. A. Coldren et al., Appl. Phys. Lett., Vol. 38, p. 315 (1981); K. J. Ebeling et al., Electron. Lett., Vol. 18, p. 901 (1982); Coldren et al., IEEE J. Quantum Elect., QE-18, p. 1679 (1982).
In all of the lasers categorized above, there exist problems in achieving efficient single longitudinal mode operation under high speed modulation conditions because of dynamic spectral characteristics of the lasers.